Our research centers on the biochemistry and function of the multicatalytic proteinase complex (MPC; proteasome), first isolated in our laboratory from bovine pituitaries as an unusually high molecular mass (- 700 kDa), multisubunit complex, involved in such fundamental cell functions as antigen processing, mitosis, and degradation of regulatory proteins such as cyclins, oncogene products, and transcription factors. Our aims are: 1) To identify components of the MPC expressing specific proteolytic functions by selective labeling of their active sites with [14 C]3.4-dichloroisocoumarin and other labeled isocoumarins, and by using specific substrates and inhibitors for protection of defiried components of the MPC from covalent modification; 2) To identify the amino acids involved in catalysis in the active centers of catalytically active subunits; and 3) to explore the mechanism involved in conversion of the latent forrn of the MPC to an active form. This work will identify subunits that express specific proteolytic functions, provide insight into the structure of the catalytic centers and the mechanistic aspects of proteolysis catalyzed by the MPC and identify factors important for conversion of the latent form to an active form of the MPC.